A. Stimulation of Escherichia coli Adenylate Cyclase Activity by Elongation Factor Tu. A unique feature of eucaryotic adenylate cyclases is their interaction with GTP-binding proteins that mediate hormonal responses. We found that the most abundant protein in E. coli, the GTP-binding protein EF-Tu, which is important as an elongation factor in protein synthesis, also serves as a stimulator of adenylate cyclase activity. B. Inhibition of E. coli Adenylate Cyclase Activity by Inorganic Orthophosphate Dependent on IIIglc of the Phosphoenolpyruvate:glycose phosphotransferase System. The relationship of adenylate cyclase, inorganic orthophosphate and the proteins of the phosphoenolpyruvate:glycose phosphotransferase system (PTS) was studied. A strain deleted for the genes for Enzyme I and HPr, IIIglc or all three proteins. The fully reconstituted strain showed a Pi-dependent stimulation of adenylate cyclase activity; in contrast, the strain expressing only IIIglc showed a Pi-dependent inhibition of adenylate cyclase activity. C. Structure-Function Analysis of cAMP-independent Forms of the cAMP Receptor Protein. The cAMP receptor protein ordinarily requires cAMP for function in operon expression. We examined the properties of three mutant forms of the protein which function in the absence of added cAMP. By cloning and DNA sequence analysis, we determined the primary structure of the mutant proteins. Additionally, we purified the mutant proteins and characterized the effect of these mutations on the secondary structure of the proteins. We also used a purified in vitro transcription system to study the properties of the proteins.